Incremental advance of plot media, such as paper, mylar, and other printable materials, requires an accurate knowledge of the position of the medium with respect to the previous advance position in order to make logically consistent prints. The knowledge of such position must be within certain minimal tolerances, in order to avoid scan-to-scan position errors, which would degrade plot quality.
Prior solutions employ expensive, high tolerance plot media rollers as well as precision gearing or belts driven by either stepper motors or DC motors/encoder systems. However, such solutions are typically limited to accuracies which result in visible print inconsistancies at the print scan boundaries and are not able at present to achieve the accuracy required to eliminate this problem. Such required accuracy is only achieved at high accuracies on the order of 0.0001 inch or less.
Other solutions employ techniques which incrementally advance plot media by using optically sensed position feedback from registration marks printed on the plot media. This method leaves the marks on the finished plot, which can be objectionable to the user.